Processes for depositing thin films of various materials on substrates are well known and have been found to be very useful. The processes can be broadly classified into two categories: physical vapor deposition (to which this invention applies) and chemical vapor deposition. As used herein, chemical vapor deposition generally refers to that coating art wherein a plurality of reactive components are introduced into a coating chamber which may or may not be evacuated. The components are caused to chemically react with one another, and the products of the reaction form a film that is coated upon the substrate. Chemical vapor deposition processes can be conducted at various pressures and temperatures.
As used herein, the term physical vapor deposition refers to that coating art wherein at least one of the coating components is initially placed into the coating chamber in a non-gaseous form. The non-gaseous coating component is generally called the "source". The coating chamber is typically evacuated to sub-atmospheric pressure prior to and during the coating process. Sufficient energy is applied to the source material to change it to vapor state, which vapor subsequently comes to rest in film form on the substrate, perhaps after combining with other components. Electrostatic and/or electromagnetic fields may be used in the process of converting the source material to its vapor phase as well as to direct the coating particles toward the substrate.
There are a number of different physical vapor deposition techniques, which are distinguished in the manner in which the source material is vaporized. One physical vapor deposition technique involves heating the source material in a crucible. The crucible is heated until the contained source material melts and then vaporizes. A related technique involves passing electric current directly through the source material so that the source melts and then vaporizes due to Joule heating. In the latter process, the electrical energy is physically conducted to the source through a metallic conductor, and an arc is not generally created.
Physical vapor deposition techniques also include ionic bombardment and sputtering deposition techniques. With these techniques, the source material is disposed within the coating chamber as a target and is bombarded with accelerated ions. The bombarding ions impart sufficient energy to the source target material to vaporize it.
Still another type of physical vapor deposition technique, and one to which this invention is particularly applicable, is that of electric arc vapor deposition. Here, as opposed to the resistive Joule heating process described above, an arc is intentionally struck, and the electrical energy contained in the art is controlled, to vaporize the source material, thus creating a coating "plasma". The source material is biased at one electric potential within the coating chamber and acts as one electrode (usually the "cathode ") of the electric arc discharge circuit. Another portion of the deposition chamber is biased at a second potential, different from the source potential, and acts as the second electrode (usually the "anode") of the electric arc discharge circuit. An arc-initiating trigger element is positioned proximate to the cathode source and is positively biased with respect to the cathode. The trigger element is momentarily allowed to engage the surface of the cathode material, establishing a current flow path through the trigger and cathode. As the trigger element is removed from engagement with the cathode source, an electrical arc is struck, which is thereafter maintained between the cathode and the anode electrodes of the chamber. In fact, a plurality of such arcs are typically maintained between the two electrodes in an operative electric arc vapor deposition chamber. This electric arc vapor deposition phenomenon is well known, and need not herein be discussed in detail. The electric arc(s) energy is sufficient to vaporize the source material, forming a coating plasma for subsequent deposition onto substrates within the deposition chamber.
One type of coating source material that has been used for the cathode in an electric arc vapor deposition system, is titanium (Ti). When a Ti source material is used, a reactive gas such as nitrogen (n.sub.2) is often introduced into the deposition chamber during the vaporization of the Ti source. The nitrogen gas reacts with the Ti to form a coating plasma within the chamber, which comprises Ti, N.sub.2, TiN and other such complexes. TiN forms a gold-colored coating that has been found to be a very durable coating for cutting tools and the like.
To assist in describing a physical vapor deposition system, a diagrammatic view illustrating a typical such system, and in particular that of an electric arc physical vapor deposition system, is illustrated in FIG. 1. Referring thereto, the deposition chamber is generally illustrated as a box-like structure. The chamber is typically evacuated (sometimes down to pressures as low as 10.sup.-6 Torr) by an appropriate vacuum pump. The substrate(s) to be coated is placed upon an appropriate substrate holder within the chamber as illustrated, and may be rotated (as illustrated by the rotation arrow) to assist in uniform coating by the coating plasma. The coating source material is appropriately mounted in the chamber so as to operatively address the substrate(s) to be coated. In FIG. 1, three such coating material sources are illustrated (one in the top wall of the chamber and two in each of the side walls of the chamber). The assembly for mounting the coating material source is typically referred to as an "evaporator", and is so designated in FIG. 1. The coating material source of each evaporator, is designed for removal and replacement, and often-times the entire evaporator assembly is configured for removal from the chamber wall. Accordingly, appropriate seals (not illustrated in FIG. 1) must be provided between the evaporator assembly and the chamber wall, and within the evaporator assembly itself, to maintain the vacuum within the deposition chamber. Such seal means form a part of the present invention, and will be discussed in more detail hereinafter. Referring to FIG. 1, an inert gas may be introduced into the deposition chamber to assist in certain phases of the process. In addition, reactive gases such a nitrogen can be introduced into the chamber during the deposition process, as illustrated in the FIGURE by the functional block termed "reactive gas". Generally, in physical vapor deposition processes, and in particular in electric arc vapor deposition processes, many of the plasma particles travel essentially in straight lines (i.e. in a line-of-sight manner) from the coating sources to the substrate(s), as generally illustrated by the straight line patterns of FIG. 1. The substrate(s) may be biased by an appropriate Power Supply, as illustrated, to attract ionic particles of the coating plasma toward the substrate surfaces.
The present invention is directed toward a mounting assembly for a coating material source of a physical vapor deposition chamber, and in particular to such a mounting assembly for a coating material source that acts as an electrode of an electric arc vapor deposition chamber. A typical prior art electrode mounting assembly is diagrammatically illustrated in FIG. 2. It will be understood that the prior art diagram illustrated in FIG. 2 is intended only for the purposes of assisting in explaining the design constraints and problems associated with such mounting structures, and is not intended to provide an exhaustive explanation thereof. In particular, only that portion of a coating material source electrode holder or mounting assembly in which the electrode functions as a cathode of an electric arc vapor deposition chamber, is illustrated. Referring to the figure, the cathode mounting assembly is generally mounted to the deposition chamber wall in a manner such that the chamber wall is electrically isolated from the entire electrode mounting assembly. Such connection may include a non-conductive acrylic ring and non-conductive washers and bushings around the mounting bolts, as illustrated. O-ring seals in combination with the acrylic ring provide a vacuum seal between the chamber wall and the electrode mounting assembly. Such electrode mounting assemblies of the prior art have typically included a mounting plate or ring, sometimes referred to a source plate, which supports a cathode shield structure (hereinafter described) and an inner cathode assembly. The cathode assembly is secured to the source plate by appropriate fastening means (fastener "A"), and is separated from the source plate by a seal, which typically has been in the form of an O-ring in the prior art. The O-ring seal provided both electrical isolation between the source plate and the cathode assembly as well as providing a vacuum seal for the internal deposition chamber. The cathode assembly mounts a replaceable cathode of the coating source material which when operatively mounted addresses the internal deposition chamber, means for cooling the cathode, and means for electrically biasing the cathode. In the apparatus illustrated in FIG. 2, a circulating cooling fluid enters a cooling reservoir through an inlet port and exits through an outlet port. The cathode is secured to and closes the open end of the cooling reservoir by means of a mounting harness and bolt that is fastened to the inner cathode assembly housing near the biasing end of the assembly. In the FIG. 2 illustration, the spacing between the upper cathode surface (and cathode shield) and the source plate is greatly exaggerated, for illustration purposes. The reservoir seal at the cathode end is achieved by an O-ring sandwiched between the cathode's lower surface and the end of the housing. The other end of the reservoir is sealed by means of an end cap threaded to the inner cathode assembly housing and an O-ring disposed between the end cap and the housing. An electrode biasing post or wire is fastened to the reservoir end cap and provides an electrical connection for a power supply for biasing the electrode through the housing and cathode mounting harness. Minimal cooling of the O-ring seal at the reservoir end cap end is provided by perforations through the inner cathode assembly housing adjacent the cathode mounting harness bolt, which allow some cooling fluid to leave the reservoir and engage the end cap and associated O-ring. The cathode assembly also typically includes a magnet, generally in the form of an electromagnet, usually mounted external of the deposition chamber and generally referred to as a "spot coil".
The spot coil provides a magnetic field in relation to the cathode during the deposition process, which causes the cathode spots to more uniformally traverse the upper surface of the cathode source material so that the cathode erodes in fairly uniform fashion during the deposition process. The cathode shield functions to maintain the cathode spots on that surface or face of the cathode which is directed toward the deposition chamber cavity and to prevent them from drifting off of such face and down the sides of the cathode. The cooling fluid is typically water, which is directed toward the rear surface of the cathode, as illustrated. Such cooling is intended to keep the sealing rings cool for preventing heat damage thereto and to minimize formation of droplets or agglomerations of the source material during deposition, to provide a smoother coating of the substrate and to provide material dimension stability for the dissimilar metals of the system.
Though the prior art cathode mounting structure such as represented in FIG. 2 generally serves its intended purpose, it suffers from several drawbacks. One area that has been particularly troublesome with the prior art structure has been with its seal members. In particular, problems have been associated with the acrylic ring and seal member(s) that have been used to isolate the chamber wall from the source plate ring and to seal the interface between the two components. As illustrated in FIG. 2, the inner edge of the insulator ring has typically been directly exposed to the inner cavity of the deposition chamber and has not been sufficiently protected from exposure to the coating plasma. Accordingly, the ring has tended to become coated by the coating plasma, which is typically of electrically conductive material. Such coating thus creates an electrical conductive path across the ring, shorting out its electrical insulation properties. In order to maintain electrical isolation between the chamber wall and the source mounting plate, such ring member has had to be removed and scraped on a regular basis, typically after each deposition run. Such cleaning, besides involving significant effort and time, also gives rise to possible damage to the ring itself during the cleaning process. Further, premature coating of the ring can limit the operative duration of a run.
Similar problems have also existed with the O-ring seal members of prior art structures. While the physical vapor deposition process is generally a line-of-sight process, there are a significant number of atoms and particles of the coating plasma that get by one or two-bounce optical barriers before condensing on a surface. With prior art structures such as illustrated in FIG. 2, the gap between the source plate ring and the inner cathode assembly mounted thereto is generally large enough to allow a significant number of particles to drift through the gap and coat the exposed surface of the O-ring seal therebetween. If a significant number of such particles coat the O-ring seal, a conductive path can be established across the seal, as was the case with the acrylic ring member. Since the inner cathode assembly and the source plate are biased at different electrical potentials, the conductive path thus formed across the O-ring destroys the electrical isolation between the two assemblies. Further, on occasion the cathode spot drifts off of the face of the cathode and past the cathode shield. Such drifting cathode spots readily move down the slide of the cathode and the cathode mounting assembly and to the O-ring, itself, burning and damaging the O-ring and its sealing properties. Another problem with such O-ring structures of the prior art is that they are generally not self-centering. Such O-rings must be maneuvered and deformed so as to precisely register with the O-ring groove before they can be forced into the groove. Once forced into the groove, the O-ring preferably remains in the groove while the cathode assembly is positioned and secured to the source plate mounting ring. Typically, such cathode mounting assemblies are disposed such that the source plate is oriented in a vertical plane. If the O-ring groove is made deep enough so as to adequately house the O-ring so that it will remain in place during the cathode assembly positioning process, there is a high likelihood that the O-ring will collapse under the mounting pressure such that the cathode assembly comes into electrical contact with the source plate ring. On the other hand, if the O-ring groove is made too shallow, the O-ring has a tendency to fall out of the groove during the positioning process.
Still another drawback of such prior art electrode mounting assemblies relates to the construction of the electromagnet or spot coil. As mentioned above, the magnetic field generated by the spot coil is intended to cause the cathode to erode more uniformally, thereby maximizing the useful life of the cathode. Prior art electromagnetic spot coils have generally been quite heavy and difficult to install when the source plate assembly is in a vertical orientation, such as illustrated in FIG. 2. Further, such electromagnetic spot coils require separate electrical power supplies and wiring, both of which are subject to failure. Additionally, the electromagnetic spot coil windings have been vulnerable to the high temperatures and abuse to which they are subjected in operative use, causing such windings to short out to the source plate ring and typically required physical removal thereof when performing routing maintenance operations on the assembly.
Another drawback with prior art cathode mounting assemblies has been that the source plate assembly of such structures has typically been bolted to the chamber wall (as illustrated in FIG. 2) in a manner requiring registration of the bolts with the bolt holes prior to final assembly. Such procedure has generally been awkward and time consuming, and requires the assembler to divert his attention from the critical seal elements during the alignment procedure. Further, when the source plate assembly is removed from the chamber, to provide access to the internal cavity of the chamber or to replace the cathode source, the assembly has to be set aside where it can easily be damaged. Bolts and nuts used to secure the source plate assembly can easily be lost or misplaced during the disassembly/assembly process.
The present invention is directed to the aforementioned problems. The insulator and seal member of the prior art devices are replaced by insulator ring and seal construction that is less vulnerable to coating. Such construction decreases the maintenance required and enables the deposition machine to be used for multiple "runs" before the insulator ring need be cleaned or replaced. Likewise, with the configuration of this invention, insulator seals are much less likely to become coated by the source material or to be come operatively damaged by a drifting arc spot. The insulator rings of the present invention also have the advantage of being self-centering, in the sense that they register more readily with the source plate ring and cathode assemblies, and in that they also assist in the self-centering of other components of the source plate assembly. The insulator rings of the present invention are desiged so that they will remain in place once positioned, and do not fall from or out of grooves as have the O-ring seals of prior art devices. The insulator rings of the present invention thereby reduce the "down time" attributable to maintenance, facilitate cathode source replacement and allow for faster multiple-cycling time of the deposition machine.
The present invention also replaces the electromagnetic spot coil of prior art designs with a compact simple permanent magnet structure that alleviates the problems associated with such unreliable prior art structures. The source plate assembly of the present invention is better cooled, so as to resist agglomeration of the coating source plasma particles. In a preferred embodiment of the present invention, the loose bolts and nuts formerly used to interconnect the chamber wall and source plate assemblies have been eliminated, further minimizing the amount of time required to reassembly the source plate assembly prior to operation of the deposition machine. These and other advantages of the invention will become apparent to those skilled in the art, upon a more detailed description of a preferred embodiment of the invention.